A gun sight is a device used to provide an accurate point of aim for firearms such as rifles, handguns and shotguns. Sights are used on other types of weapons such as bows and crossbows as well. Popular gun sights include the traditional metallic sights (also known as “iron sights”) and telescopic sights (also called “rifle scope” or “scope” for short). Other types of sights include red-dot sights, holographic sights, laser sights, etc.
Traditional metallic sights are inexpensive, sturdy and light in weight. However, the shooter is required to line up the rear sight with the front sight and the target. With a telescopic sight, he simply lines up the sight's cross hairs (reticle) with the target. It is very difficult for the human eye, if not impossible, to try to switch its focus from a rear sight to a front sight to a target as required with metallic sights, and its frustrating to say the least. Telescopic sights eliminate this frustration. Furthermore, most telescopic sights also magnify, making the target appear closer, and therefore easier to see, enabling the shooter to place a more precise shot on the target.
A telescopic sight can dramatically improve the functionality of a firearm by providing the shooter with a simple yet highly accurate means for aiming at distant targets. Current designs, however, do not completely eliminate the requirement that the shooter must align his eye with the sight.
A first disadvantage of current telescopic sights is that if the shooter's eye is not aligned with the optical axis of the sight, he will not see the complete field of view. This problem becomes more prominent in high-magnification scopes where even a slight misalignment of the eye from the optical axis can cause the target image to partially or completely black out. In high stress situations, this makes fast target acquisition very difficult.
A second, more severe, disadvantage of telescopic sights is that the shooter might also misplace the shot due to a phenomenon called parallax error. Telescopic sights use an objective lens to form an image of the target on the reticle. The exact location of this image depends on the target distance. Therefore, for long and short shots, the target image is focused either in front of or behind the reticle. If the image is not coplanar with the reticle, then putting one's eye at different points behind the sight's ocular (eyepiece lens) causes the reticle to appear to be at different points on the target (see FIGS. 1a and 1b). This phenomenon is referred to as parallax error.
Parallax error is a serious issue as it prevents a shooter from aiming accurately if he happens to be looking at an angle into the sight. Most manufacturers design their telescopic sights to be free of parallax error at a fixed distance, say 100 yards. However, if the target is at any distance greater than or less than 100 yards, which is often the case, there is potential for misplacing the shot as the parallax error may cause the shooter to aim at an offset distance from the intended target point.
A telescopic sight which uses an adjustable objective (AO) to compensate for parallax error has been disclosed in U.S. Pat. No. 2,858,732 issued Nov. 4, 1958 to E. O. Kollmorgen, et. al. By adjusting the objective, one can focus the image of targets located at various distances exactly on the plane of the reticle and eliminate parallax error. Other prior art on scopes with adjustable objective include U.S. Pat. No. 3,336, 831 issued Aug. 22, 1967 to J. Unertl, Jr. and U.S. Pat. No. 4,072,396 issued Feb. 7, 1978 to C. J. Ross and W. R. Weaver. Currently, several manufacturers (e.g., Schmidt and Bender GmbH of Biebertal, Germany) make scopes with adjustable objectives where they are also designated as Parallax Adjustable (PA), Side Focusing (SF), etc.
A major drawback with AO scopes is that to adjust the objective for parallax-free aiming, the shooter must either know the exact distance to the target or use a process of trial and error. That is, he must position his scoped firearm on a steady platform so that it is aiming at the target without any movement. Then, without touching the gun or the scope, move his head from side to side while looking through the scope. If the reticle moves around on the target, the shooter is seeing parallax error. He then needs to adjust the objective and repeat this experiment until he observes no reticle movement when he moves his head behind the scope. Clearly, this is a time consuming and often impractical task to do in the field.
Another approach to deal with parallax error has been contemplated in U.S. Pat. No. 6,865,022 issued Mar. 8, 2005 to S. J. Skinner and S. D. Moore. This patent teaches an improved reticle that can give the shooter visual indication when his eye is not properly aligned with the optical axis of the scope. This may help detect parallax error but it doesn't actually eliminate it. The shooter is still required to perfectly align his eye with respect to the optical axis of the sight. Therefore, this design is not a solution to the problem.